Re: RAID 6 rebuild time - can I slash it?

With those I/O numbers yes 0% is correct!

Take drive 1 with only 3.4 average requests per second averaging 7K it's
seriously loafing.

With numbers like these RAID-6 will likely have little effect. As you
say though the 5 hour time means these numbers are averaged to mush. The
10 minute slice is better. Your idea to do this during billing is also
a good one. You might actually find a time slice where a minor slowdown
could occur. :-)


- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com

On 3/20/2014 12:27 PM, Jeff Crosby wrote:

*This message was transferred with a trial version of CommuniGate(r) Pro*
Well, here's an up-to-date WRKDSKSTS. Note it's over 5 hrs worth.

Work with Disk Status
DILGARD
03/20/14
12:09:59
Elapsed time: 05:22:55



Size % I/O Request Read Write Read Write
%
Unit Type (M) Used Rqs Size (K) Rqs Rqs (K) (K)
Busy
1 4326 35165 48.2 3.4 7.5 .9 2.5 11.4 6.0
0
2 4326 26373 48.2 2.2 8.9 1.0 1.2 10.8 7.3
0
3 4326 26373 48.2 2.3 8.0 .8 1.4 11.6 6.0
0
4 4326 26373 48.2 3.6 7.2 1.1 2.4 10.4 5.7
0
5 4326 26373 48.2 2.9 7.9 .9 2.0 11.9 6.1
0
6 4326 35165 48.2 3.6 8.2 1.2 2.4 12.6 6.0
0
7 4326 35165 48.2 5.2 7.5 1.5 3.7 11.9 5.7
0
8 4326 35165 48.2 3.5 8.6 1.3 2.2 11.9 6.6
0
9 4326 30769 48.2 3.3 8.5 1.3 1.9 11.6 6.5
0
10 4326 30769 48.2 3.2 8.2 1.1 2.0 11.8 6.1
0
11 4326 30769 48.2 3.4 7.9 1.1 2.2 11.3 6.1
0
12 4326 30769 48.2 2.9 7.7 .9 2.0 11.7 5.9
0
13 4326 30769 48.2 3.6 7.5 1.1 2.5 11.6 5.7
0
14 4326 30769 48.2 3.1 8.1 1.1 1.9 11.1 6.4
0
15 4326 30769 48.2 2.8 7.8 .9 1.9 11.4 6.0
0
16 4326 30769 48.2 3.9 8.9 1.5 2.4 10.7 7.8
0

0% Busy for every drive? The help for % Busy says "This field is blank

if

the performance characteristics of the
disk unit are not available." Is zero the same as blank on this display?
I wouldn't think so, but really don't know.

I then did F13 to restart statistics. Let it run for 10 minutes. All
drives still show 0% with comparable numbers to the above in the other
columns.

I think I will look at it later today while the billing is running and

see

if I get at least a 1% somewhere.






On Thu, Mar 20, 2014 at 12:08 PM, DrFranken <midrange@xxxxxxxxxxxx>

wrote:

It Depends. :-)

Seriously it really does.

If (which is impossible) you had 100% reads then your I.O times will be
approximately 12.5% slower (1 of 8) because of the hot spare not being
available to 'contribute'.

If (also impossible) you had 100% writes then you should see more than
50% longer response times as there are two additional ops required plus
the hot spare not contributing and don't forget the RAID card will be
more busy tracking two RAID sets.

But of course you do a mix of reads and writes, more reads less penalty
more writes more penalty.

And then there is queuing theory to deal with. If your disks are
currently 3 to 5% busy you're not going to care much if they go to 5 to
8% busy. But if they are already 35 to 40% busy and jump to 50 to 60%
busy that's gonna hurt.

From everything you've said I really don't think you will be in

trouble

here, just better protected!

- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com

On 3/20/2014 12:01 PM, Jeff Crosby wrote:

*This message was transferred with a trial version of CommuniGate(r)

Pro*

But, Jeff, if your run time went from 38 seconds to 5 minutes, would

they

complain?

Is that what's going to happen? Everything will be slower by a factor

of 7

or 8 *times* (38 seconds to 300 seconds)? Seriously, or are you saying

it

for effect?




On Thu, Mar 20, 2014 at 11:44 AM, <rob@xxxxxxxxx> wrote:

Everything is relative. Many of us on this list will go on and on

about

how much of a performance dog a 3 drive raid set is on scsi disks. As
recently stated: PA-THE-TIC! However one poster put on here (some

years

ago) that these people were coming from a S/36 and wouldn't really

know

how fast the machine could go. They would be impressed with what they
got.

But, Jeff, if your run time went from 38 seconds to 5 minutes, would

they

complain?


Rob Berendt
--
IBM Certified System Administrator - IBM i 6.1
Group Dekko
Dept 1600
Mail to: 2505 Dekko Drive
Garrett, IN 46738
Ship to: Dock 108
6928N 400E
Kendallville, IN 46755
http://www.dekko.com





From: Jeff Crosby <jlcrosby@xxxxxxxxxxxxxxxx>
To: Midrange Systems Technical Discussion <

midrange-l@xxxxxxxxxxxx>

Date: 03/20/2014 11:03 AM
Subject: Re: RAID 6 rebuild time - can I slash it?
Sent by: midrange-l-bounces@xxxxxxxxxxxx



In various discussions regarding disk and performance and RAID 6 going
back
many months, posters keep hammering away on the performance of RAID 6.

The

most important thing to us is protection.

Last October, Dr Franken posted this:
http://archive.midrange.com/midrange-l/201310/msg00754.html

I assume that's still true.

Regarding our performance, we have no performance issues, whatsoever.

I

mean none. No one *ever* complains about response time or how long a
batch
job takes on the System i. Ever.

As an example, we're a food distribution warehouse. The orders to be
delivered today were staged yesterday and then are run all-at-once in
batch. The operator took the option at @4:35pm. This processed every
order against inventory, generated all picking labels, the invoices,

and

all documentation needed by the night crew to load the trucks. This
entire
process took 38 seconds. This is very typical. The actual *print*

time

for those labels and invoices was obviously longer, but the job was

done.

This is followed by running a ton of reports (some hardcopy and some

PDF)

in a follow up batch job. This takes less than 2 minutes.

The above is our most complex and important business hours task, and

it's

less than 3 minutes total. Should I be worried about performance?




On Thu, Mar 20, 2014 at 10:39 AM, Wilson Jonathan
<piercing_male@xxxxxxxxxxx

wrote:

On Thu, 2014-03-20 at 08:33 -0400, rob@xxxxxxxxx wrote:

Sue,

Raid 6 would mean you'd have to lose at least two drives to be wiped

out.

Raid 6 with hot spare means you'd have to lose at least three

drives,

right?

Raid 6 means you can loose 2 working disks, but can continue running,
but if 1 more working disk failed during a re-building spare you're

dead

in the water, unless it was the rebuilding spare in which case you're
still running.

Kind of like, which is better:
Raid 5 with a hot spare, or
Raid 6 with no hot spare
Either way, you'd have to lose two drives to be fried. I would

think

that

Raid 6 would be better if there was a risk the second drive would be

lost

while the hot spare for raid 5 was becoming active and still

rebuilding.

With Raid 5 you lose the space of one drive to striping (spread out

across

all drives). How much do you lose to Raid 6?

You loose 2 drives worth of space to the redundancy, no matter the
layout or number of raid6 disks.

Ideally the parity should be spread across all the disks as apposed

to

specific parity drives, at least as far as linux mdadm is

concerned...

it may be different for IBM i cards/systems but in the linux world

its

much faster to have the parity interleaved with the data as it

prevents

contention on what would become a parity disk(s).

Is there a performance degradation from removing one of your disk

drives

of a 8 drive SCSI raid set to become hot spare?
I would think there would be two performance hits. One, dropping

from

8

drives supporting the raid stripe down to only 4. The other

performance

hit would be one less disk arm assisting while it's just sitting

there

waiting to be hot spare.


Rob Berendt
--
IBM Certified System Administrator - IBM i 6.1
Group Dekko
Dept 1600
Mail to: 2505 Dekko Drive
Garrett, IN 46738
Ship to: Dock 108
6928N 400E
Kendallville, IN 46755
http://www.dekko.com





From: Sue Baker <sue.baker@xxxxxxxxxx>
To: midrange-l@xxxxxxxxxxxx
Date: 03/19/2014 06:34 PM
Subject: Re: RAID 6 rebuild time - can I slash it?
Sent by: midrange-l-bounces@xxxxxxxxxxxx



Jeff Crosby <jlcrosby@xxxxxxxxxxxxxxxx> wrote on Wed, 19 Mar
2014 16:29:52 GMT:

On a Saturday of my choosing, our HW service provider is going
to take our System i 520 from 2 8-drive RAID 5 sets to 2
8-drive RAID 6 sets w/hot spare.

Wow! This is quite a change in data protection.

You realize that in order to have your data at risk, you need to
lose 2 drives in the RAID6 array?

Generally, I recommend to customers to do RAID5 + hot spare or
RAID6. I see it as more importante to have as many disk units
as possible for RAID6 versus adding the tiny bit of reduced
delay time for rebuild to begin on a single failed drive that is
provided by hot spare.

Time to create the RAID6 arrays is mostly dependent on the size
of the drives. Other factors are whether there is data on them
or not (% full is irrelevent) and the type of disk controller.

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--
Jeff Crosby
VP Information Systems
UniPro FoodService/Dilgard
P.O. Box 13369
Ft. Wayne, IN 46868-3369
260-422-7531
www.dilgardfoods.com

The opinions expressed are my own and not necessarily the opinion of

my

company. Unless I say so.
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